chapter 2. dosimetric principles, quantities and units - IRSN

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Chapter 2. Dosimetric Principles, Quantities and UnitsAs a high energy photon beam penetrates the medium, collision kerma is maximalat the surface of the irradiated material because photon fluence is greatest at thesurface. Initially, the charged particle fluence, and hence the absorbed dose,increases as a function of depth until the depth of dose maximum z m axis attained.• If there were no photon attenuation or scattering in the medium, but yetproduction of electrons, a hypothetical situation, as illustrated in Fig. 2.3(a),would occur: the build-up region (with β < 1) is followed by a region of completeCPE where D = K col, i.e., β = 1.• In the more realistic situation, however, due to photon attenuation and scatteringin the medium, a region of TCPE occurs (Fig. 2.3(b)) where there exists anessentially constant relation between collision kerma and absorbed dose. Thisrelation is practically constant since, in high-energy photon beams, the averageenergy of the generated electrons and hence their range does not changeappreciably with depth in the medium.FIG. 2.3. Collision kerma and absorbed dose as a function of depth in a medium, irradiatedby a high-energy photon beam.48
Review of Radiation Oncology Physics: A Handbook for Teachers and Students• In the special case where true charged particle equilibrium exists (at the depth ofmaximum dose in the medium), the relation between absorbed dose D and totalkerma K is given by:D= Kcol = K(1 − g), (2.25)where g is the bremsstrahlung fraction, depending on the electron kinetic energy;the higher the energy, the larger is g . The bremsstrahlung fraction also dependson the material considered, with higher values of g for higher Z materials. Forelectrons produced by cobalt-60 gamma rays in air the bremsstrahlung fractionequals to 0.0032.• The build-up of absorbed dose is responsible for the skin sparing effect in the caseof high-energy photon beams. However, in practice the surface dose is small butdoes not equal zero because of the electron contamination in the beam due tophoton interactions in the media upstream from the phantom or due to chargedparticles generated in the accelerator head and beam modifying devices.2.7.4. Collision kerma and exposure• Exposure X is the quotient of dQ by dm, where dQ is the absolute value of thetotal charge of the ions of one sign produced in air when all the electrons andpositrons liberated or created by photons in mass dm of air are completelystopped in air:XdQ= . (2.26)dmThe unit of exposure is coulomb per kilogram (C/kg). The special unit used forexposure is the roentgen R, where 1 R = 2.58 x 10 -4 C/kg. In the SI system of units,roentgen is no longer used and the unit of exposure is simply 2.58×10 -4 C/kg ofair.• The average energy expended in air per ion pair formed W air is the quotient of Eby N, where N is the mean number of ion pairs formed when the initial kineticenergy E of a charged particle is completely dissipated in air:WairE= . (2.27)NThe current best estimate for the average value of W air is 33.97 eV/ion pair or33.97 × 1.602 × 10 19 J/ion pair:Weair−19( ion pair) × × ( )−191.602×10 ( C / ion pair)33.97 eV / 1.602 10 J / eV= =33.97 J/C .(2.28)49
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